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Chemical Engineering Journal 226 (2013) 409–415
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Chemical Engineering Journal
journal homepage: www.elsevier .com/locate /cej
Treatment of an agrochemical wastewater by integration ofheterogeneous catalytic wet hydrogen peroxide oxidation androtating biological contactors
1385-8947/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.cej.2013.04.081
⇑ Corresponding author. Tel.: +34 91 488 71 82; fax: +34 91 488 70 68.E-mail addresses: [email protected] (M.I. Pariente), [email protected]
(J.A. Siles), [email protected] (R. Molina), [email protected] (J.A. Botas),[email protected] (J.A. Melero), [email protected] (F. Martinez).
M.I. Pariente a, J.A. Siles b, R. Molina a, J.A. Botas c, J.A. Melero a, F. Martinez a,⇑a Department of Chemical and Environmental Technology, ESCET, Universidad Rey Juan Carlos, 28933 Móstoles, Madrid, Spainb Department of Inorganic Chemistry and Chemical Engineering. University of Córdoba, Campus Universitario de Rabanales, Edificio Marie Curie, Ctra. N-IV, km 396,14071 Córdoba, Spainc Department of Chemical and Energy Technology, ESCET, Universidad Rey Juan Carlos, 28933 Móstoles, Madrid, Spain
h i g h l i g h t s
� Agrochemical wastewater treatment by catalytic peroxide oxidation and biological system.� Oxidation reduced significantly the total organic carbon and increased its biodegradability.� Remarkable performance and stability of the rotating biological contactors.� Treated effluent fulfilled the mandatory values of the regional law.
a r t i c l e i n f o
Article history:Received 14 December 2012Received in revised form 16 April 2013Accepted 18 April 2013Available online 28 April 2013
Keywords:PesticidesAgrochemical wastewaterCatalytic wet hydrogen peroxide oxidationRotating biological contactorsBiodegradability
a b s t r a c t
The treatment of a non-biodegradable agrochemical wastewater has been studied by coupling of heter-ogeneous catalytic wet hydrogen peroxide oxidation (CWHPO) and rotating biological contactors (RBCs).The influence of the hydrogen peroxide dosage and the organic content of the wastewater (dilutiondegree) were studied. The CWHPO of the raw wastewater at 80 �C and using a moderate amount of oxi-dant (0.23 gH2O2/gTOC) reduced significantly its total organic carbon content and increased its biode-gradability. Likewise, the iron leaching of the heterogeneous catalyst (Fe2O3/SBA-15) was less than2 mg/L in the treated effluent. Under the best operating conditions, the resultant CWHPO effluent wassuccessfully co-treated by rotating biological contactors (RBCs) using a simulated municipal wastewaterwith different percentages of the CWHPO effluent (2.5, 5 and 10% v/v). The RBCs showed high stability forthe treatment of the highest percentage of the CWHPO effluent, achieving total organic carbon (TOC) andtotal nitrogen (TN) reductions of ca. 78% and 50%, respectively. The integration of both processes on acontinuous mode has been successfully accomplished for the treatment of the as-received agrochemicalwastewater.
� 2013 Elsevier B.V. All rights reserved.
1. Introduction
There is a large scale development of pesticides during the lastfew decades due to its fundamental role in food and fibre produc-tions. In recent years, the presence of pesticides in water and food[1] and their adverse effects on the human health and the equilib-rium of the ecosystems [2] have paid the attention of differentinvestigations [3]. The monitoring of these compounds over thelast 20 years have highlighted some chronic effects such as carci-nogenesis [4], neurotoxicity [5], sterility [6] and cell development
effects, particularly in the early stages of life [7]. These substancesare present in wastewaters coming from agrochemical plants ded-icated to the manufacture of pesticides and industries of food man-ufacturing. Their critical effect on the environment and humanhealth makes necessary the degradation of these compounds byefficient and friendly-environmental technologies [3].
Biological processes are the most attractive treatments in termsof economic costs and environmental concerns. However, they donot always provide satisfactory results, especially for the treatmentof industrial wastewaters, since many of the containing organicsubstances are toxic or resistant to biological degradation [8,9].In this sense, other kind of technologies such as advanced oxida-tion processes (AOPs) has emerged as promising alternatives forthe treatment of industrial wastewaters with highly refractoryand toxic pollutants [10,11]. These processes are based on the
Table 1Agrochemical wastewater characterization before and after the CWHPO treatment.Operation conditions: T = 80 �C, s = 11.6 min, pH0 = 3, [H2O2]0 = 0.23 gH2O2/gTOC.
Parameter Industrialwastewatera
Oxidizedwastewatera
Limitvalueb
COD (mg O2/L) 28067 ± 722 13693 ± 431 1750TOC (mg/L) 9040 ± 440 4539 ± 609 –BOD5 (mg O2/L) 2100 ± 205 1860 ± 82 1000BOD5/COD 0.07 0.13 –pH 5.7 ± 0.1 3.2 ± 0.1 6–10NO�3 (mg/L) 13 ± 4 11 ± 8 –NO�2 (mg/L) <0.01 <0.01 –NT (mg/L) 688 ± 57 605 ± 50 125
a Number of samples analyzed = 3.b Emission limit value according to Comunidad de Madrid Legislation.
410 M.I. Pariente et al. / Chemical Engineering Journal 226 (2013) 409–415
generation of non-selective and highly reactive hydroxyl radicals(HO�), which are ones of the most powerful oxidation agents [12].
Among the AOPs, several works have been published for thetreatment of wastewaters containing pesticides, using UV radia-tion alone or in presence of hydrogen peroxide [13] or TiO2 as het-erogeneous photocatalysis [14]. In other cases, ozonation orultrasonic-based processes have been also used [15]. In all thecases, the efficiency of these technologies was significant limitedto low pollutant concentrations.
Other AOP that is often employed for the treatment of moreconcentrated effluents is Fenton technology [16]. The oxidationof different pesticides by means of Fenton-like reactions usingferrous salts and hydrogen peroxide has been studied in numer-ous works during last years [17]. Nowadays, Fenton technologyis commercially used to treat different kind of industrial waste-waters [18]. To overcome typical limitations of homogeneousFenton catalytic systems, strong pH dependence ranging ca. from2.5 to 3.5 and recovery of dissolved iron species as sludge in afinal separation step, several types of heterogeneous Fenton-likecatalysts have been developed in the last decades [19–23]. In par-ticular, our research group has synthesized a catalyst based onsupporting crystalline iron oxides over a mesoporous SBA-15 sil-ica matrix [20]. This catalyst has been successfully used for thedegradation of phenol as model pollutant [24,25] and an indus-trial wastewater [10], using temperatures between 80 and120 �C as so-called catalytic wet hydrogen peroxide oxidation(CWHPO) process.
The operating conditions of CWHPO systems are normally fo-cused on the mineralization of the organic loading [26]. In thissense, the economical feasibility of CWHPO is often questionedby the high oxidant consumption and the energetic costs of theoperation temperature. However, from other point of view,CWHPO can be also used at milder operation temperatures andmoderate oxidant dosages, as pretreatment obtain a more biode-gradable effluent for a subsequent biological process instead ofincreasing the TOC mineralization [27].
For the coupling of CWHPO and biological processes, the appli-cation of heterogeneous catalysts in CWHPO is also a crucial point.As compared to homogeneous systems, the heterogeneouscatalysts prevent additional metal pollution and therefore, costlyseparation units for its recovery, making simpler the integrationof both processes. As far as we know, few works are described inliterature coupling heterogeneous CWHPO processes and biologi-cal systems [28,29].
In the studies of coupling processes, the biological treatment isnormally based on suspended growth batch reactors as activatedsludge-like processes [30]. However, limited work has been car-ried out using attached growth reactors such as rotating biologicalcontactors (RBCs). Recently, several works have shown the poten-tial of RBCs for the treatment of different complex wastewaters,due to the large active surface and high biomass concentration,which allow high organic loading rates and short hydraulic reten-tion times (HRT) with a robust stability of the process [31,32]. Inthis sense, a palm oil mill effluent (POME) was treated in an acti-vated sludge system using a diluted raw wastewater (5000 mg/Lof COD concentration) with a HRT of 36 h, obtaining a CODremoval efficiency of 42% [33]. In contrast, RBCs were able to treata concentrated POME (ca. 23500 mg/L of COD concentration)obtaining a COD removal efficiency higher than 70% for the sameHRT [34].
In this work, the treatment of a non-biodegradable wastewaterfrom an agrochemical plant has been studied by coupling of heter-ogeneous catalytic wet hydrogen peroxide oxidation (CWHPO) androtating biological contactors (RBCs) as biological treatment. Theheterogeneous CWHPO system has been evaluated in terms ofincreasing the biodegradability of the agrochemical wastewater
for feasible co-treatment of the CWHPO effluent with a simulatedmunicipal wastewater in the subsequent RBCs. Up to date, thisfully integration of heterogeneous CWHPO with a biological treat-ment is considered one the first approaches to the treatment of realwastewaters with a high stability of both heterogeneous catalyst ofadvanced oxidation process and biological treatment.
2. Materials and methods
2.1. Agrochemical wastewater characterization
The studied wastewater was taken from an agrochemical man-ufacturing industrial plant located in Madrid (Spain), which is ded-icated to the synthesis of different kind of pesticides. Thecharacterization data of the as-received wastewater is shown inTable 1.
2.2. CWHPO experimental set-up
The CWHPO system consists of a fixed bed reactor made of glasswith an inner diameter of 1.2 cm and 15 cm of length. As catalyst,crystalline iron oxides supported over a mesoporous SBA-15 silicamatrix was used. The catalyst was conformed as extrudates of2.0 mm � 1.5 mm following a methodology described elsewhere[25]. The pellets show crystalline hematite entities of ca. 14 wt.%of iron content and the main properties of SBA-15 topology, suchas mesoscopic order and narrow pore diameter distribution, witha BET surface of about 264 m2/g. The catalyst particles are packedbetween glassy beads to enable a better distribution of the inletsolution inside the catalytic bed. A schematic view of the CWHPOexperimental set-up is shown in Fig. 1a.
The wastewater feed was acidified until a pH value of ca. 3 byaddition of a sulphuric acid solution 1 M. After addition of appro-priate amount of hydrogen peroxide, it was pumped by a Gilson10SC HPLC pump to the reactor. The temperature of the reactorwas controlled by the circulation of a heating fluid (silicone)along an external jacket. The residence time and temperaturewere kept constant according to preliminary studies using thesame experimental set-up at 11.6 min and 80 �C, respectively[10,25]. The residence time was calculated according to Eq. (1),being wCAT the catalyst weight and QF and qF the volumetric flowand the density of the feeding stream, respectively. Samples fromthe treated effluent were withdrawn across the experiments inorder to monitor the performance of the CWHPO runs. Prior tocoupling experiments, the feasibility of treating the as-receivedwastewater without dilution and the optimal hydrogen peroxidewere studied.
s ¼ wCAT
Q F � qFð1Þ
(a)
(3)(b)
(5)(4)
(2)
(1)
Fig. 1. Schematic experimental set-up of integrated CWHPO and RBC processes: (a) CWHPO reactor for the treatment of the raw agrochemical wastewater; and (b) RBCs usedfor the biological treatment.
M.I. Pariente et al. / Chemical Engineering Journal 226 (2013) 409–415 411
2.3. Rotating biological contactor system (RBC)
The rotating biological contactors (RBCs) at lab-scale were usedfor the biological treatment of the previously oxidized wastewater.The bioreactor is made of AISI 304 stainless steel and has a totalvolume of 5.25 L with five discs. The diameter of each disc is30 cm and the total surface area for biomass attachment is0.71 m2, being around 40% of each disc submerged in water. Thedisc rotational speed was 30 rpm. During the preliminary start-ing-up step and prior to the combination with the CWHPO effluent,the organic loading rate was 2 gTOC/day m2. A schematic represen-tation of the RBC experimental setup is shown in Fig. 1b. The RBCswere inoculated with biomass coming from an urban wastewatertreatment plant located at Rey Juan Carlos University (Spain) ofca. 2600 inhabitants equivalent, which is operated by RBCs as sec-ondary treatment with 17 gCOD/day m2 of organic loading rate, andca. 200 mg/L and 60 mg/L of COD in the inlet and outlet effluents,respectively. Thereafter, the RBCs were fed batch-wise for 25 days[35] with a synthetic municipal wastewater [36] that contains pep-tone (160 mg/L), meat extract (110 mg/L), urea (30 mg/L), K2HPO4
(28 mg/L), NaCl (7 mg/L), CaCl2 (4 mg/L) and Mg2SO4 (2 mg/L)and sucrose (100 mg/L) as additional carbon source until the for-mation of an homogeneous biofilm attached to the RBCs. After thatperiod of time, the RBCs were continuously fed with the same sim-ulated wastewater previously indicated but without sucrose for66 days.
2.4. CWHPO and biological integration
Once the RBCs were stabilized and acclimatized, the integrationof the CWHPO with the RBCs was carried out. In the integrated sys-tem, the effluent of the CWHPO was fed with the synthetic muni-cipal wastewater at different volumetric ratios (2.5%, 5% and 10%)and constant feed flow rate of 10 mL/min. As the process was fullycoupled and the hydraulic retention time of RBCs was maintainedconstant (8.75 h), a higher production of the CWHPO effluent wasneeded when the volumetric ratio was increased. In order to pro-vide a higher volumetric flow of the treated CWHPO effluent, theCWHPO feed flow rate was increased, but also with the catalystmass (0.25, 0.5 and 1 mL/min with 2.9, 5.8 and 11.6 g, respec-tively). Thus, the residence time of the catalytic fixed bed reactor,as defined in Eq. (1), was maintained constant for the three volu-metric ratios (2.5%, 5% and 10%).
An overall schematic representation of the integrated experi-mental set-up of the CWHPO and the RBCs for the treatment ofthe agrochemical wastewater is shown in Fig. 1. The performanceof the integrated system was evaluated by analysis of the samplestaken from the simulated municipal (1) and industrial wastewater(2) tanks, the CWHPO effluent (3), the RBC feed (4) and treatedeffluent (5).
2.5. Analytical methods
Total organic carbon (TOC) content of the solutions was ana-lyzed using a combustion/non dispersive infrared gas analyzermodel TOC-V Shimadzu, which was calibrated with a standardsolution of potassium phthalate prior to the TOC analyses. Metam-itron, one of the herbicides detected in the composition of thewastewater, was analyzed using a HPLC chromatograph (VarianProstar) equipped with a Waters dC18 column and an UV detector.Measurements of UV–Vis absorption carried out at 220 nm in aVarian Cary 500 Scan UV–Vis–NIR spectrophotometer were alsoused as indirect indicator of pollutants with chromophore groupssuch as aromatic rings, which are characteristic of herbicide com-pounds. For the treated solutions of the CWHPO runs, the conver-sion of the hydrogen peroxide was determined by iodometrictitration and the iron content was measured by inductively cou-pled plasma-atomic emission spectroscopy (ICP-AES) analysis col-lected in a Varian VISTA AX system. Chemical oxygen demand(COD) and biochemical oxygen demand (BOD) were determinedfollowing the procedures described by APHA [37]. On the otherhand, NO�3 , NO�2 , total nitrogen (TN) and SO2�
4 were measured bycolorimetric methods (Maxidirect, Lovibond) at different wave-lengths [37].
Aerobic biodegradability of the as-received wastewater andsamples after CWHPO treatment were assessed by Zahn–Wellensbioassays [38]. Typically, 0.2 L of sample with a COD concentrationof ca. 1000 mg/L is firstly neutralized with NaOH up to a pH of 7.0.Thereafter, 4 mL of sludge with ca. 2 g/L of suspended solids com-ing from the urban wastewater treatment plant located at Rey JuanCarlos University, was added to the previous solution and shakenat 150 rpm under diffuse illumination at room temperature. Thebiodegradability of samples was estimated from the COD removalfor 17 days. As biodegradable reference sample, sucrose as carbonsource was used following the same procedure above mentioned.
6050
45
50 40
40
35
30
30 25
X(%
)
20 15
20
10XTOC
(%)
XH2O2
(%)
[Fe]le
achin
g(m
g/L
)
10
5
0 0[Fe] leaching (mg/L)
0 2000 4000 6000 8000 10000
[TOC]0
(mg/L)
Fig. 2. CWHPO runs of agrochemical wastewater with different dilution degrees.Operation conditions: T = 80 �C, s = 11.6 min, pH0 = 3, [H2O2]0 = 3.5 g H2O2/g TOC.
3
6
9
12
15
18
[Fe]le
achin
g(m
g/L
)
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
0
20
40
60
80
100
0.0
0.2
0.4
0.6
0.8
1.0X
TOC(%)
XH2O2
(%)
X(%
)
[H2O
2]0/[TOC]
0(gH
2O
2/gTOC)
Absorbance
Absorb
ance
(a.u
.)
Fig. 3. CWHPO runs of agrochemical wastewater using different hydrogen peroxidedosages. Operation conditions: T = 80 �C, s = 11.6 min, pH0 = 3, [TOC]0 = 9450 mg/L.
412 M.I. Pariente et al. / Chemical Engineering Journal 226 (2013) 409–415
3. Results and discussion
3.1. Evaluation of catalytic wet hydrogen peroxide oxidation process(CWHPO)
The as-received agrochemical wastewater showed a low biode-gradability according to the BOD5/COD ratio (0.07) with a relativelyhigh TOC concentration (9.4 g/L) (see Table 1). These characteriza-tion data make necessary the CWHPO treatment prior to the bio-logical treatment. In many studies, the dilution of the originalwastewaters has been used in order to achieve a good catalyticperformance. In this study, the efficiency of the CWHPO was eval-uated for the as-received wastewater at different dilution degrees.Likewise, the dosage of hydrogen peroxide is a crucial issue inCWHPO processes, particularly from the economic point of viewdue to its high consumption. In the case of using CWHPO as pre-treatment for a subsequent biological process, the influence ofthe hydrogen peroxide concentration was studied in order to max-imize its efficiency in the production of a biodegradable effluentand to minimize the amount of oxidant unconverted after reaction.
Thus, preliminary experiments were carried out with inletstreams of TOC contents ranging from wastewater samples tentimes diluted (0.9 g/L) up to the as-received wastewater withoutdilution (9.4 g/L). Temperature, oxidant dosage and residence timewere kept constant at 80 �C, 3.5 gH2O2/gTOC and 11.6 min, respec-tively. As seen in Fig. 2, a constant TOC conversion was achievedregardless of the inlet organic content. However, the hydrogen per-oxide conversion reached values of 60% for the most diluted inletstream, decreasing until 26% for the as-received wastewater. Tak-ing into account that the initial oxidant/TOC ratio was constantfor all the experiments (3.5 gH2O2/gTOC), the decrease of thehydrogen peroxide conversion could be attributed to the catalystdeactivation by strong adsorption of organic compounds whenhigher TOC concentrations are used. On the other hand, the similarTOC conversions, even though with different oxidant consump-tions, seem to indicate the mineralization of the organic matterby thermal degradation in some extension. This fact was confirmedby a blank experiment at the same temperature but in absence ofcatalyst and hydrogen peroxide. In terms of the TOC conversion,the performance of the blank experiment was slightly lower thanthe CWHPO one. However, the CWHPO experiment revealed aremarkable degradation of metamitron (85%), one of the majortoxic pollutants detected in this wastewater, as compared to theblank experiment with negligible degradation.
The influence of the inlet TOC concentration on the iron leach-ing of the heterogeneous catalyst in the liquid phase was also eval-uated (Fig. 2). As can be seen, the iron concentration of the outleteffluent for as-received wastewater was ca. 17 mg/L. In turn, a sig-nificant decreasing was gradually observed as the wastewater wasdiluted. This is in agreement with the hypothesis that organic sub-strate plays an important role on the catalyst leaching. Therefore,aromatic compounds seem to be responsible of reductive–oxida-tive reactions over the iron sites of the catalyst that promotes itsdissolution in the liquid phase [25]. Other studies in literature havepostulated that some by-products could form complexes with thesupported iron [39], being responsible of the iron leaching.
In order to make the CWHPO economically attractive, the use ofsmall oxidant dosages is recommended. Additionally, when a sub-sequent biological treatment is considered after CWHPO, theamount of oxidant should be limited to degradation of toxic and/or non-biodegradable pollutants to oxidized by-products amenableto biological treatment. At the same time, the amount of uncon-verted oxidant must be reduced in order to preserve the microor-ganism population. In this study, several experiments were carriedout varying the hydrogen peroxide concentration in the as-re-ceived wastewater feed stream from 3.5 to 0 gH2O2/gTOC, whereas
residence time (11.6 min), temperature (80 �C) and acidification ofthe wastewater were kept constant. Fig. 3 shows the results ofthese experiments according to different variables: the removalof TOC, the hydrogen peroxide conversion, UV–Vis absorbance at220 nm, and the iron concentration in the outlet effluent for thedifferent hydrogen peroxide/TOC ratios.
The influence of the hydrogen peroxide was not very significantin terms of TOC mineralization, achieving values of ca. 55% even inthe absence of the oxidant. However, some interesting results wereobtained taking into account other characterization data related tothe composition of the residual organic content. Thus, it was seenthat a minimum hydrogen peroxide dosage (0.23 gH2O2/gTOC)yielded a significant reduction of the UV–Vis absorbance (charac-teristic of pesticide pollutants containing aromatic compounds)and metamitron. The increase of the oxidant concentration onlyshowed a beneficial reduction of metamitron from 34% to 85%.Concerning the catalyst stability, the lowest amount of ironleaching was attested for the smallest oxidant dosage, with iron
0 2 4 6 8 10 12 14 16 18
0
20
40
60
80
100Sucrose
CWHPO wastewater
Industrial wastewater
Bio
degra
dabili
ty(%
)
Time (days)
Fig. 4. Zahn–Wellens biodegradability assays of wastewaters samples before andafter the CWHPO treatment.
Table 2Characterization data of the RBC streams during the starting up period.
Parameter RBC feeda Treated effluenta
COD (mg/L) 250 ± 25 56 ± 6TOC (mg/L) 108 ± 11 10 ± 3pH 7.0 ± 0.2 6.0 ± 0.1NO�3 (mg/L) 1.0 ± 1.2 17 ± 14NO�2 (mg/L) 0.25 ± 0.02 0.27 ± 1.24NHþ4 (mg/L) 57 ± 15 39 ± 11Turbidity (NTU) 12 ± 1.5 1.6 ± 1.3
a Number of samples analyzed = 3.
M.I. Pariente et al. / Chemical Engineering Journal 226 (2013) 409–415 413
concentrations below 2 mg/L, which is a crucial point not only forthe life of catalyst but also for the subsequent integration with thebiological treatment. Regarding the oxidant conversion, the maxi-mum conversion (nearly 95%) was achieved using the lowest oxi-dant dosage.
According to these results, 0.23 gH2O2/gTOC was considered theoptimum oxidant dosage for CWHPO of the agrochemical waste-water as pre-treatment prior to final biological treatment. For thisoxidant dosage, further characterization such as the BOD5 andCOD, the conductivity, the turbidity and the nitrogen content indifferent forms were assessed. Table 1 shows the characterizationdata of as-received wastewater and the CWHPO effluent (permit-ted values of some parameters according to the current legislationwere also included). Biodegradability profiles of both samples forZahn–Wellens bioassays were also evaluated (Fig. 4). From allthese results, it was able to determine the increase of the biode-gradability of the CWHPO effluent. Zahn–Wellens bioassays ofthe CWHPO effluent reached a biodegradability of ca. 60% after17 days as compared to the negligible biodegradation of as-re-ceived wastewater. Although this biodegradability is lower thanthat obtained by the sucrose solution, used as highly biodegradablecontrol sample, this value is close to the threshold described in lit-erature for considering a wastewater biodegradable (c.a. 70%) [38].In agreement with Zahn–Wellens bioassays, the BOD5/COD ratioafter the CWHPO treatment increased from 0.07 to 0.13. Therefore,although the CWHPO effluent cannot be considered completelybiodegradable, the mixture of this effluent with a highly biode-gradable urban wastewater would be an interesting alternativefor co-treatment the CWHPO effluent.
0 5 10 15 20 25 300
20
40
60
80
100
120
140
[TO
C](m
g/L
)
Time
Fig. 5. TOC profiles of the simulated
3.2. Integrated CWHPO–RBCs treatment
Prior to the integration of CWHPO with RBCs, the latter biolog-ical treatment was operated for 10 weeks under continuous feed-ing of a synthetic municipal wastewater with a hydraulicretention time of 8.75 h. The organic loading rate was 2 gTOC/day m2 with an inlet stream of ca. 100 mg/L of TOC concentration.Fig. 5 shows TOC concentration of the inlet and outlet streamsalong the acclimation time, where it can be seen the high stabilityof the system. During that period of time, TOC removals higherthan 90% were achieved, whereas the reduction of the COD was77%, as it is seen in Table 2. Additionally, the oxidation of the nitro-gen nutrients was also attested.
Fig. 6 depicts the TOC concentration of inlet agrochemicalwastewater (2), CWHPO effluent (3), RBC feed (4) and RBC treatedeffluent (5) along the operation time for the three percentages ofthe CWHPO effluent in the RBC feed (2.5, 5 and 10% v/v). TheTOC profiles evidenced a high stability of the performance of bothchemical and biological processes. For the lowest percentage (2.5%of CWHPO effluent and 97.5% of the simulated municipal wastewa-ter), the TOC conversion was ca. 55% after the CWHPO and 66%after the subsequent RBCs. These results indicate a good adaptationof the aerobic microorganisms for the by-products of the CWHPOeffluent, which accounts for ca. 53.6% of the TOC content of the in-let RBC stream. Thus, it is remarkable that RBCs are able to biode-grade not only the TOC fraction of the simulated municipalwastewater but also part of the TOC loading of the CWHPO efflu-ent. As the percentage of the CWHPO effluent in the RBCs feedwas increased a remarkable enhancement of the TOC removalwas observed (71% and 78% of TOC removals for 5% and 10% ofthe CWHPO effluent, respectively). Note that mixing percentagesof 5% and 10% correspond to TOC loadings coming from the treatedagrochemical wastewater of 70% and 83% of the overall TOC con-tent in the RBC feed. Thus, there is no doubt of the successful per-formance of the RBC for the treatment of a non-biodegradableagrochemical wastewater previously treated by CWHPO to reduce
35 40 45 50 55 60 65 70
Treated Effluent(5)
RBC Feed (4)
(days)
wastewater in the RBC system.
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70
0
200
400
600
3000
6000
9000
12000
(10 %)(5 %)(2.5 %)
Treated Effluent(5)
RBC Feed (4)
CWHPO Effluent (3)
Industrial Wastewater (2)
[TO
C](m
g/L
)
Time (days)
Fig. 6. TOC profiles of the integrated CWHPO–RBC treatment.
414 M.I. Pariente et al. / Chemical Engineering Journal 226 (2013) 409–415
the organic loading and to increase its biodegradability. These re-sults are in clearly agreement with the BOD5/COD ratio and theZahn Wellens bioassays of the CWHPO effluent.
Additionally, other parameters such as COD, nitrates, nitrites,total nitrogen, sulphates and pH were determined in the finalstreams of integrated CWHPO–RBC treatments for 5% and 10% ofthe CWHPO effluent in the RBC feed. Table 3 summarizes the char-acterization data of the RBC feed and treated effluent at the pseu-do-steady state. This pseudo-steady state was characterized byconstant effluent characteristics measured in terms of TOC andCOD when three consecutive readings had variation less than±10% [40]. COD removals were also improved as the percentageof the CWHPO effluent in the RBC feed increased (71% and 80%for the 5% and 10% of the CWHPO effluent in the RBC feed, respec-tively). For the nitrogen compounds, it was observed an increase ofthe NO�3 and NO�2 concentrations. Likewise the overall total nitro-gen (TN) content decreased during the biological treatment (55%and 49% for the 5% and 10% of the CWHPO effluent in the RBC feed,respectively). These results indicate both autotrophic nitrificationof the nitrogen sources via ammonium oxidation and heterotro-phic denitrification of nitrite and nitrate. The nitrites/nitrates pro-duced in the outer aerobic biofilm layer could diffuse into deeperanoxic layers of the biofilm and there, they could be reduced inparallel with ammonium oxidation. Consequently, nitrificationmay occur on the surface of the biofilm, while denitrificationwould take place in the inner layers of the biofilms as anoxic partof the RBCs where the oxygen diffusion is limited along the thick-ness of the biofilms [32,41].
Finally, it must be point out that the final effluents taken fromthe CWHPO–RBC system comply all the monitor parameters of
Table 3Characterization data of the inlet and outlet RBC streams for integrated CWHPO–RBCprocess using 5% and 10% of CWHPO effluent in the RBC feed.
Integration Parameter RBC feeda Treated effluenta
5% COD (mg/L) 831 ± 88 242 ± 78TOC (mg/L) 353 ± 29 103 ± 28pH 7.5 ± 0.2 7.7 ± 0.1NO�3 (mg/L) 1.3 ± 1.02 29.4 ± 24.8NO�2 (mg/L) 0.01 ± 0.02 1.06 ± 1.0 4TN (mg/L) 120 ± 15 54 ± 22
SO2�4 (mg/L) 211 ± 19 98 ± 26
10% COD (mg/L) 2057 ± 274 401 ± 75TOC (mg/L) 677 ± 119 146 ± 25pH 7.3 ± 0.1 7.9 ± 0.2NO�3 (mg/L) 2.9 ± 2.5 8.0 ± 5.1NO�2 (mg/L) 0.02 ± 0.02 1.01 ± 0.85TN (mg/L) 113 ± 13 58 ± 8
SO2�4 (mg/L) 221 ± 80 107 ± 25
a Number of samples analyzed = 3.
the Spanish legislation that regulates the discharge of liquid indus-trial effluents to the sewage system: COD (401 mg/L), pH (7.7–7.9),TN (58 mg/L) and SO2�
4 (107 mg/L).
4. Conclusions
An agrochemical wastewater has been successfully treated bycoupling of heterogeneous CWHPO and RBCs. It has been demon-strated that the heterogeneous CWHPO may be part of a processtrain for increasing the biodegradability of an agrochemical waste-water. The CWHPO process was able to convert the non-biodegrad-able compounds of the as-received wastewater into oxidized by-products which are more amenable to be biologically degradedby the consortia of RBCs. In the integrated CWHPO/RBCs process,the RBC system was fed with 2.5%, 5% and 10% in volume of theCWHPO effluent. The highest percentage of CWHPO effluentshowed a remarkable performance of the RBCs with TOC and TNreductions of 78% and 49%, respectively. Moreover, a high stabilitywas also proven by RBCs for the co-treatment of a simulated mu-nicipal wastewater with up to 10% of the CWHPO effluent.
Acknowledgements
The authors thank ‘‘Comunidad de Madrid’’ and European SocialFund through the Project S2009AMB-1588 and the Spanish Minis-try of Science and Innovation for the financial support through theproject Consolider-Ingenio 2010 CSD2006-044 and for funding JoséÁngel Siles López through Project CTM2005-01293 and Grant BES-2006-14074. Authors also thank ACAI Depuracion S.L. (Huesca,Spain) for the transfer of the RBC.
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